In the field of shipping containers there is a series of containers called platform-based containers comprising a rectangular platform base. Sometimes these have erect corner posts at either end and are then known as flatracks. The bases of the platforms comprise a framework of longitudinal beams and transverse members.
Hot rolled I beams used to be used in the construction. However these were found too heavy and it is now more common for the beams to be fabricated from steel plate and sheet. The fabrication technique is to weld the pieces of steel together to form an I section beam similar to the earlier hot rolled type. Added to the beams are stake pockets, lashing devices, recesses for folding posts and so on. Thus, there can be a significant amount of heat generated by welding and cutting resulting in an accumulation of residual stresses in the beams.
After the beam is built into the platform base, the base is fitted out with a deck, typically timber and thus presented for carrying cargo. The platforms are mostly supported at their end corners such that as cargo is loaded onto the base, the base naturally deflects downwardly.
It has been found that when loaded the beams can deflect significantly more than established theory predicts, and on removal of the load, the beams are found permanently deformed. Such deformation is not acceptable. It has been suggested that the cause of the deformation is that the combined residual stress plus the bending stress caused by the load exceeds the elastic limit of the steel, and thus the steel yields.
To overcome this problem, it is usual to increase the strength of the beams. So one method is to use steel with a much higher yield point so that perhaps the combination of bending stress plus residual stress still falls under the elastic limit of the steel. This saves weight of steel but adds the cost of higher strength steel.
Alternatively, the beams can be reinforced with extra steel to reduce the bending stress once more adding cost, but also weight.
Another device is to stress relieve the beams allowing residual stresses to dissipate. Stress relieving can be done by applying heat to the base such as occurs when the bases are sometimes hot dip galvanized, or by shot blasting or peaning the surface, or by vibration.
Alternative residual stresses can be kept sufficiently low by careful welding to keep heat to a minimum, and by using stress relieved steel plate.
However all these methods are expensive, inconvenient for fast commercial production and the resulting geometry of the bases cannot easily be controlled from one team of welders or batch of steel to another.